1. Field of the Invention
The invention relates to a method for manufacturing a monolithic plastic board with a thickness of at least 6 mm, preferably (20.+-.10) mm, which board is constructed as structural foam and comprises a sandwich-like structure with a foam core and closed skin plates.
2. Description of the Prior Art
Such a method and board obtained therewith is known for instance from U.S. Pat. No. 4,096,218.
Known from this American patent specification is a method according to which a few seconds after completion of injection of the plastic the mould cavity is enlarged in a predetermined time and in controlled manner. By making use of a foaming agent and application of gas counter pressure can thus be achieved that the foam structure is of good quality, while the skin of the obtained board has a closed and smooth structure.
According to the said American patent the mould is held for a predetermined time in its closed starting position, whereafter the mould undergoes a determined expansion in a subsequent specified period. After reaching maximum expansion cooling takes place for a time, whereafter the formed board can be removed.
Attention is also drawn to U.S. Pat. No. 4,133,858, which deals with a related technique and wherein mention is also made of a number of possible plastics for use in the context of this technique and substances such as foaming agents for adding thereto.
Since it has been found that with the known method boards of a sufficiently high quality cannot be obtained, it is an object of the invention to provide a method with which a board of improved quality can be manufactured.
A further object of the invention is to perform the known method such that it lends itself to prior placing in the mould of a decorative foil such that this foil is positioned on an outer surface of the obtained board.
Yet another object of the invention is to perform the method such that the obtained board has an optimum flatness within very narrow tolerances or, more generally, has a form which within very narrow tolerances is the same as the desired nominal form.
Yet a further object of the invention is to perform the method such that less expensive plastics, for instance plastics of recycled quality, are also usable to obtain a board with the desired superior properties.